Heretofore, fused rather than agglomerated fluxes have been used in pipe mills because of superior bead shape, penetration, and minimal undercutting obtained in this application. The reason fused fluxes perform better under these conditions is that a smaller percentage of the total heat input is consumed in melting a given mass of fused flux than agglomerated flux. In addition, less filler metal is deposited at a given current using a fused flux than with an agglomerated flux. As a result, when using a fused flux, a smaller percentage of the heat input is comsumed in melting the flux and the filler metal and a larger percentage of the heat input is available to superheat the deposited weld metal and base plate. This translates directly into increased penetration and an improved bead contour (low profile) resulting from an increase in the time required for the weld bead to solidify.
In addition to improving bead contour, the longer weld metal solidification time allows the weld pool to flow more readily to completely fill the crater cut by the welding arc. This greatly minimizes any "undercutting" tendency. This is especially advantageous for multi-arc, high speed procedures such as are used in the seam welding of pipe.
Also another desirable feature of fused flux is that it is possible to regrind slag and reuse it with little loss of performance. The reground slag of agglomerated fluxes in the past have had an excessively high melting range and will not perform satisfactorily in this application.
Fused fluxes have the disadvantage over agglomerated fluxes in that they contain a significant amount of potent glass formers such as SiO.sub.2. Its use at levels usually above 30% ensure the low melting glass composition required for ease of manufacture and proper melting range for welding. These glass formers, however, are usually acidic and impart an acid character to the flux. Hence, the flux has a low basicity index which can result in the weld metal exhibiting poor notch toughness. In addition, acidic fluxes tend to lose manganese to the slag during welding while building silicon in the weld metal. Because it is extremely difficult to make alloy additions to fused fluxes as explained hereinafter, it is often necessary to use expensive high manganese electrodes with these fluxes to ensure the proper weld deposit chemistry (Mn and Si) for good notch toughness and crack resistance.
Agglomerated flux has the following inherent advantages over fused flux. It is possible to easily add deoxidizers and other metallic alloys to agglomerated flux for improved resistance to porosity and to control the final weld deposit chemistry while using a low alloy electrode. These additions are not readily made to fused fluxes as they tend to be broken down and/or oxidized during the flux manufacturing process and rendered useless. Therefore, alloy additions must be made by using expensive alloyed electrodes.
Also agglomerated fluxes are inherently lower cost than fused fluxes because they are easier to manufacture and consume less energy to produce.